Propellant compositions



3,132,978 PROPELLANT COMPOSITIONS Arch Chilton Scurlock, Alexandria,Va., assignor to Aflantic Research Corporation, Alexandria, Van, a

corporation of Virginia No Drawing. Original applicationNov. 6, 1957,Ser. No. 694,897. Divided and this application June 22, 1962,

, Ser. No. 205,182 e 19 Claims. (Cl. 149-18) "This invention relates tonew heterogeneous monopropellant compositions capable of generatinggases containing high available energy for such purposes as producingthrust or power, heat energy or gas pressure. More specifically, itrelates to heterogeneous monopropellant compositions which combine theadvantages of liquid and solid propellants and eliminate many of theirdisadvantages. This is a divisional application of Arch C. Scurlockapplication, Serial No. 694,897, filed November 6, 1957, now Patent No.3,095,334 issued June 25, 1963. a i

The term monopropellant refers to a composition which is substantiallyself-suflicient with regard to its oxidant requirements .asdistinguished from bipropellants where the fuel is maintained separatelyfrom the oxidizer source until admixture at the point of combustion. Byheterogeneous is meant a two-phase system wherein a finelydivided, solidoxidizer is dispersed in a liquid fuel in 'which the oxidizer isinsoluble.

1 Mobile liquid monopropellants, namely propellants which are injectableinto a combustion chamber in the form offinely-divided droplets orsprays, possess a number of important advantages. The mass burning rateof the propellant and, thereby, the volume of combustion gases'produced,are controllable by varying the rate of injection. Combustion canbe'stopped by shutting off flow andresumed at will. Performance is notdependent upon the temperature environment of the system. Duration ofoperation is limited only by capacity of the storage tanks orreservoirs. Combustion chambers need be one storage tank, one propellantpump and one set of .feed. lines and valves, and eliminate elaboratesystems .to various parts of the system, such as valves. When used in arocket motor, there is some tendency for un- -U States {large enoughonlyto provide sufficient space for com- -pletion of the combustionreaction. Liquid monopropellants, furthermore, possess an importantadvantage ;over liquid bipropellants since the former require onlycharacterized by disadvantages such as low density, low specificimpulse, high toxicity, excessive sensitivity to heatand shock resultingin detonation, and corrosiveness burned droplets of the liquidpropellant to leave the combustion chamber and to be cooled duringexpansion in the nozzle before combustion occurs. Performance may alsobe affected by attitude of the system.

Not only is a complex system of tubing and valves and from there intothe combustion chamber, but provision must be made to purge it ofpropellant after test firings a're made. Metal catalysis problems aresometimes encountered in passing the liquid through the complex tube andvalve system. Catalyst beds are required required to fill the mobileliquid monopropellant tanks for combustion of some liquid propellantsand vibration of the system often poses problems of retaining the bedfirmly fixed in the combustion chamber. Storage and transportation ofliquid propellants is also a problem becaus'of its tendency to leakreadily. Such leakage presents both a fire and toxicity hazard.

at other operating conditions.

otherwise cooled Solid propellants possess the advantages of highdensity, low heat and shock sensitivity, good stability, longstorageability, absence of leakage, low corrosiveness and toxicity andelimination of propellant filling and injection equipment and controlssince all of the solid propellant is contained directly'in thecombustion chamber. Solid propellants do not require purging of thesystem after test firing, do not need an external combustion catalystand are not'affected by the attitude of the system.

Such propellants do, however, possess a number of disadvantages. Thesolid grain must be sufficiently strong and free from mechanical flawsso that it does not crack or shatterunder pressure or vibrationalstresses. Many solid propellants also tend to become excessively brittleat low ambient temperatures and'thereby subject to fracture. Cracking orshattering of the propellant grain in the combustion chamber may causesuch an enormous, uncontrolled increase in burning surface that thewalls of the combustion chamber cannot withstand the pressure. Althougha burning solid propellant grain can be quenched, necessary, by suitablemeans, reignition is not feasible, so that the unburned portion is atotal loss and intermittent operation is impractical. Ambienttemperature of the propellant grain is an important parameter indetermining burning rate and cannot be compensated for during use byvariation of the area of burning surface. I

Solid propellant grains must be predesigned with respect to burningsurface area for each particular application, since such area is set fora given grain and cannot subsequently be varied. This makes necessarythe production and storage of a large variety of grains of differentdesign. Such predesigned solid propellant grains cannot accommodateduring burning to variations in operational requirements or to differentambient temperatures. The only way in which a solid propellantgasgenerating composition can be designed to meet unforeseen operationalneeds is to design it to produce an adequate supply of gases at theextremes of high usage requirements and low ambient temperature, whichin most cases necessitates. venting and wasting surplus gas Wastage inthis manner can be as highas of the gas produced and provides a designproblem in terms of a modulating valve which can withstand the hightemperature exhaust gases. Size of the grains must also be predeterminedand permits no subsequent variation in amount consumed unless waste of.an unburned portion of the grain poses no economic or .other problem;Maximum duration of burning time or .thrust is considerably shorter thanthat of liquid propellants which is limited largely only by storagecapacity of the reservoir.

The combustion chamber must be of sufiicient size to accommodate all ofthe propellant and, therefore, is generally larger than required forcombustion of a propellant. Since the walls of the entire combustionliquid chamber must be strong enough to withstand the high combustiongas pressures and completely insulated or to-withstand thehighcornbustion gas temperatures, this may pose a more serious Weightproblem than that of a propellant storage tank. The geometry of thecombustion chamber is furthermore, immobilized by the designrequirements ofthe propellant grain and cannot, in many cases, beadapted to the particular structural needs of the device as a whole.

The object of this invention is to provide stable heterogeneousmonopropellant compositions which are characterized by high density,high autoignition temperature, and substantial freedom fromshock-sensitivity, corrosiveness and toxicity and which, furthermore, byvirtue of their physical characteristics, make possible controlledfeeding of the propellant into a combustion chamber, a

strip controllable burning surface not dependent upon subdivision oratomization in the combustion chamber,

quenching and reignition, non-leakage and tolerance of ployed togenerate gases having high available energy 7 for developing thrust orpower as, for example, for use in jet or rocketreaction motors, gasturbines, reciprocating engines, and thelike or for providingheat or gaspressure.

Other objects and advantages of my invention will be come obvious fromthe following detailed description. Broadly speaking, the monopropellantcompositions of my invention comprise a stable dispersion of'fmely-diavided, insoluble solid oxidizer in a continuousmatrix of a non-volatile,substantially shoclcinsensitive liquid fuel, the composition havingsuliiciently high cohesive strength to form a plastic mass whichmaintains the solid oxidizer in stable, uniform dispersion and which,while capable of continuous flow at ordinary to reduced temperaturesunder stress, nevertheless retains a formed shape for an appreciablelength of time. The compositions, which preferably are soft gels,possess characteristics of non- Newtonian liquids, namely yield to flowonly under a finite stress.

Such plastic, shape-retaining compositions can be fed at ambienttemperatures under pressure from a storage chamber into a combustionchamber in the form of any desired continuous coherent shape such as acolumn, or the like, with combustion taking place on the leading face ofthe advancing material.

The liquid fuel can be any, oxidizable liquid which meets the followingspecifications:

(a) It preferably comprises at least 50% by weight of a stable inertmaterial which is insensitive to shock or impact and requires anexternal oxidizer for com-' ,bustion. Thuis at. least half of the liquidfuel should not contain combined oxygen as, for example, the form ofnitroso, nitro, nitrite, or nitrate radicals, which is available foroxidation of other components of the molecule, such as carbon, hydrogen,silicon, or sulfur. It may, however, and often preferably does, containcombined oxygen which is not available to any appreciable extent forfurther oxidation, such as oxygen which is linked to a carbon, silicon,sulfur or phosphorus atom in the molecule. Up to by weight, preferablyless than 30'or 40%, based on'the liquid'vehicle, of an active,nonvolatile liquid fuel containing combined oxygen available forcombustion, such as nitroglycerin, di

mm. Hg atl00 C.

(c) The liquid: fuel should be mobile, namely freeethylene glycoldinitrate, pentaerythritol trinitrate or 1,2,4-

flowing, at ordinary temperatures, preferably having a maximumsolidification or pour point temperature of about -2 C. or less. In somecases, for example, a maximum pour point aslowas -60 C. may bedesirable. T he desirable specific, maximum solidification temperatureis' determined largely by ambient temperatures at point of use of thepropellant compositions.

The use of at least 50% of an inert, shock insensiformulation.

generally is about 8%, usually about 10% by weight.

tive liquid fuel, makes possible the formulation of fuelsolid oxidizermonopropellant compositions which have very low sensitivity to shock orimpact. The compositions are also substantially insensitive to heat,minimum autoignition temperatures generally being at least about C.,namely well above; any environmental temperatures likely to beencountered.

Substantial non-"olatility ensures extended storageability even atrelatively high environmental temperatures without loss by vaporizationof the fuel component. This 1 is essential not only to maintain thepredesigned combustion characteristics of the monopropellant but also toretain its desired physical characteristics. Vaporization of sufficientof the liquid fuel to leave a solid, granular mass would make themonopropellant unfit for the desired mode of use.

The liquid fuel must be mobile at ordinary to reduced temperatures tomake possible the desired plasticity of the monopropel-lant mixture atambient temperatures of use and to prevent freezing of themonopropellant at relatively low ambient environmental temperatures ofuse into a non-plastic solid mass. Solidification of the compositionduring storage or shipping at freezing tem peratures is of no concern solong as, ambient temperature at time of use is above the solidificationtemperature since plasticity and extrudability is restored at the highertemperature.

The inert liquid fuel can be any oxidizable liquid 7 which forms gaseouscombustion products, preferably an organic liquid which in addition tocarbon and hydro- 7 ethylene glycol, etc.; ethers, e.g., methylalpha-naphthyl ether; ketones, e.g., benzyl methyl ketone, phenylo-tolyl ketone, isophorone; acids, e.g., Z-ethyl-hexoic acid, ca-

proic acid, n-heptylic acid, etc.; aldehydes, e.g., cinnamaldehyde;nitrogen-containing organic compounds such as amines, e.g.,N-ethylphenyl-amine, tri-n-butylamine, diethyl aniline; nitriles, e.g.,caprinitrile; phosphorus-containing compounds, e.g., triethyl phosphate;sulfur containing compounds, e.g., diethyl sulfate,pentarnethyl-siloxane-methyl methacrylate, and many others.

In many cases, I prefer to employ an inert liquid fuel containing someoxygen, which, although not available for combustion, as is the case incarbon-linked oxygen, reduces the 'stoichiometric oxygen requirementsfor the remainder of 'the fuel molecule. In the case of such carbonandhydrogen-rich fuels as hydrocarbons, it is sometimes difficult toincorporate sufficient solid oxidizer to permit stoichiometriccombustion levels without fonn ingthe composition into an undesirablesolid, granular mass unsuitable for my purpose. However, stoichiometricproportions, although generally desirable in rocket motorsf is notalways essential in other-gals generator applications. In fact, reducedoxidation levels are often preferred in gas generators producing powerrather than thrust because of equipment design problems posed byexcessively high temperatures.

The amount of liquid fuel vehicle in the composition is critical onlyinsofar as anadequate amount must be the particular solids dispersed,their shape and degree of subdivision and can readily be determined byroutine test The minimum amount of liquid required Beyond the requisiteminimum any desired proportion of additivessuch as gelling agents.cohesiveness and'plasticit'y are obtained by proper size distribution ofthe finely divided solid without an addi- 'liquid fuel to dispersed'solid can be employed, depending on the desired combustion properties,since the desired cohesive, shape-retentive properties can be obtainedby Where the requisite tional gelling "agent, the amount of solidincorporated should be suflicient to provide the consistency essentialfor shapeeretentiveness. This will vary with the particular liquidvehicle, the particular solid and its size distribution and can readilybe determined by routine testings.

The solid oxidizer can be any suitable, active oxidizing agent whichyields oxygen readily for combustion of the feul and which is insolubleinthe liquid fuel vehicle. Suitable oxidizers include the inorganicoxidizing salts, such as ammonium, sodium, potassium and lithiumperchlorate or nitrate, metal peroxides such as barium peroxide andthelike. The solid oxidizer should be finely divided, preferably with amaximum particle size of about 300 to 600 microns, to ensure stable,uniform dispersion of the oxidizer in'the liquid fuel, so that it willnot separate or sediment despite lengthy storage periods, al-

though some somewhat larger particles can be maintained in gelledcompositions without: separation.

As aforementioned, the monopropellant compositions of my inventioncomprise stable dispersions of finely divided, insoluble solid oxidizerin a continuous matrix of a liquidfuel and possess suflicient cohesivestrength to retain a fo'rrned'shape while retaining sufiicientplasticity to' new continuously'at ordinary to reduced temperature underpressure. Such a two-phase system, namely a solid uniformly dispersed ina continuous liquid matrix en-f sures smooth, continuous coherent flowand, very importantly, a. constant mass burning rate for a given area ofexposed burning surface. In this respect the burning .propertiesaresimilar to those of a solid propellant grain.

A solid, granular material produced, for example, by absorption of aliquid fuel on solid particles Without formation of a continuous matrix,is not suitable for my purpose since such a material, although possiblyextrudible under high pressures, does not possess suflicient cohesivestrength to prevent breaking up of the extruded material, particularlyunder vibrational, accelerational, or other stresses in the combustionchamber and does not present a continuous, even burning surface becauseof intergranular spaces. 7 a

The shape-retentive cohesiveness of the monopropellant material shouldpreferably be sufliciently high so that it possesses a minimum tensilestrength of about 0.01 p.s.i. and preferably about 0.03 p.s.i. orhigher. The material should, however, be capable of yielding tocontinuous flow at ordinary to reduced temperatures under stress orpressure. The use of excessively high pressures to produce the requisteflow is undesirable for practical reasons, although availablepressure-producing devices will of course, vary with particularapplications. The maximum shear stress at a wall required to initiateand sustain flow 'of the composition at ordinary or ambient temperaturesis preferably not higher than about 1 p.s.i. with a maximumof about 10p.s.i.

Compositions having thixotropic properties, namely compositions, which,above a certain minimum stress, tend to increase in fluidity withincreasing stress and to i 4 decrease in fluidity with decreasingstress, are particularly suitable for my purpose, especially sincefrictional or shearing stresses exerted on the surface of the extrudingmaterial by the walls of the injection tube or orifice tend to enhancefluidity at the fractional interface and to improve flow properties.

A composition having some thixotropic properties can be made byincorporating suflicient finely divided solid, insoluble oxidizer intothe liquid fuel to make an extrudab'le mass,"when the particles are sodistributed that the minimum ratio of size of the largest to thesmallest parti- 'cles is about 2:1 and preferably about 10:1. At'least90% of the particles by weight should preferably have a maximum size ofabout 300 microns. Above this, a small proportion by weight up to about600 microns can be tolerated. There is occasionally an undesirabletendency, however, for such compositions to separate after a period ofseveral days. p Generally, I have found it desirable to impartthixotropic properties by incorporating a gelling agent in the solidoxidizer-liquid fuel dispersion. Such gels possess the desireddispersion stability, cohesiveness, shape-retentiveness and flowcharacteristics. Any gelling agent which forms a gel with the particularliquid fuel can be employed. Examples of gelling agents compatible withmany of the non-volatile liquid fuels include natural and syntheticpolymers such as polyvinyl chloride; polyvinyl acetate; celluloseesters, e.g., cellulose acetate and cellulose acetate butyrate;cellulose ethers, e.g., ethyl cellulose, and carboxymethyl cellulose;metal salts of higher fatty acids such as the Na, Mg and Al stearates,palmitates and the like; salts of naphthenic acid; casein; paraya gum;gelatin; bentonite clays, and amino-treated bentonite clays; etc.Although this isnot essential, I prefer to employ organic gelling agentssince they can also function as fuels during combustion. The amount ofgelling agent employed is largely determined by the particular liquidfuel, the particular gelling agent, and the specific physical propertiesdesired. The amount of finely divided solid present also is adeterminative. factor since, broadly speaking, the smaller the amount'ofdispersed solid, the larger the amount of gelling agent'required.Particle size distribution of the dispersed solids is generally. not anirnportant factor in imparting cohesive, plastic properties-to thecomposition and in minimizing separation where a gelling agent isemployed since these factors are adequately provided for by the gel.Even some substantially large solid particles as, for example, uptoiabout v1000 microns, can be held in stable dispersion. However, thepresence of different size particles is often desirable because of theimproved packing effect obtained, in terms of increased amounts ofsolids which can be incorporated. Y

Finely divided, solid metal powders, such as Al or Mg, can beincorporated in the monopropellant compositions asan additional fuelcomponent along with-the liquid fuel. .Such metal powders possess theadvantages both of increasing'density and improving specific impulse ofthe monopropellant because of their high heats of combustion. The metalparticles should preferably be within .a sizerange of 0.25 to 50microns. The amount of such metal fueladded is not critical but isdetermined largely by thespecific use and .the requisite physicalcharacteristics of the composition as aforedescribed. For example, itshould not be. incorporated in such.- large amounts that the mixtureeither becomes granular in texture" or deficient in amount of oxidizer.In general the maximum amount of metal powder which can be introducedwhile maintaining the desired physical properties of the composition andan adequate amount of solid oxidizer is about 45% by weight, and dependsupon the density of the metal and its chemical valence or oxidantrequirement for combustion.

contains some combined oxygen as aforedescribed, While maintaining itsessential physical characteristics.

In some cases, as for example,where the monopropellant is being employedin a gas generator for driving a stoichiometric oxidizer levels withrespect to the liqturbine, reciprocating engine, or the like, as asource' of gas pressure, or to provide heat energy, the amount ofoxidizer can be less than'stoichiometric so long as sufficient isintroduced to maintain active combustion and a desired level of gasgeneration. The presence of an active liquid fuel component, namely afuel containing oxygen available for combustion, reduces, of course, theamount of solid oxidizer required both for stoichiometric and less thanstoichiometric combustion levels.

Other additives which can be incorporated into the monopropellantcompositions include, for example, burning rate catalysts, such asammonium dichromate, copper chromite and ferric ferrocyanide; coolantsfor. reducing the temperatures of the generated gases where necessary,as in the case of some turbine applications, such as monobasic ammoniumphosphate, barbituric acid and ammo- V nium oxalate; and the like.

The heterogeneous monopropellants are easily prepared, generally bymixing at ordinary temperatures. In some cases, it may be desirable toaccelerate gelation'by heating to dissolve the gelling agent in theliquid fuel vehicle. The solid oxidizer can then beadmixed with thegelled liquid, although the order of addition is not critical.Thickening of the mixture with'the appropriate small amount of gellingagent during formulation, in this manner, produces a composition havingthe requisite;

physical characteristics aforedescribed, such as viscosity andcohesiveness, without further significant change during storage, whichwould destroy these properties and the usefulness of the compositionsfor the intended mode of application. Manufacturing operations arerelatively non-hazardous because of the low sensitivity of thecomponents. Even where aportion of the liquid fuel is a highlyactivecompound, dilution with the inert liquid reduces shockandimpact-sensitivity to an almost negligible degree and raises theautoignition point well above even unusual environmental temperatures.

EXAMPLE I v 74.2% ammonium perchlorate (a mixture of 1725 r.p.m. and14,000 r.p.m. grinds in a ratio of 1:2; size range of 4 to 400 microns,98% by weight under 300,

microns); 24.8% triacetin and 1% copper chromite wereadmixed at roomtemperature. The resulting com position was a cohesive, shape-retentivemass which could be made to flow continuously under moderate pressure.

The composition had an autoignition temperature of 275 C. and an impactsensitivity of 80/85 cm. with a 3.2 kg. weight. Burning rate of. thematerial at atmospheric pressure was 0.04 in./sec. The material was ex'truded through a stainless steel tube 0.162 inch in diameter into anitrogen-filled chamber and the leading face was burned at a rate of 0.1in./sec. at 35 p.s.i.a.

EXAMPLE II R;p.m.: Viscosity (centipoise) The reduction in viscositywith increasing spindle speed, namely increasing stress demonstrates thethixotropic properties of the material.

The composition had a linear burning rate of 0.75 in./ sec. at 1000p.s.i.a.

EXAMPLE III p A gel was made with 75% ammonium perchlorate (1725 and14,000 r.p.m. grinds, 1:2), 24%. dibutyl sebacate and 1% polyvinylchloride. The polyvinyl chloride was mixed with the dibutyl sebacate andheated to 170 C. to form a gel, which, upon cooling, was a thickened,mobile liquid. After cooling, the gelled liquid was loaded with theammonium perchlorate. The composition was a plastic, shape retentivemass having a tensile strength of 0.31 p.s.i. Length of an extrudedcolumn before breaking under its own weight was 5 inches. Shear stressat the wall required to initiate flow inra inch diameter tube was 0.035p.s.i. 7

The dispersion was highly stable as shown by vibrator at 60 cycles andan acceleration of 4 g. No separation oc curred after 185 hours. Thematerial was also tested by centrifuge at an acceleration of 800 g. andshowed no separation after minutes.

Autoignition temperature of the composition was 286 C. and itssolidification or freezing point -18 C.

The composition extruded as a shaped mass through a 12 inch tube with0.375 inch bore at a rate of 0.25 in./

sec. under a pressure of 11 p.s.i.

Linear burning rate of the material at'7 0 F. and 1000 p.s.i. was 0.46in./sec.

EXAMPLE IV V V The following gel mixtures were prepared:

NH O104 (1725 and 14,000 r.p.m. grinds1.2), percent.-. 78 Dibutylsebaeate, percent 23.5 21 Polyvinyl chloride, percent. 1.5 1

Both compositions were plastic, cohesive, shape-retentive gels. Gel Ahad an autoignition temperature of 276 C., gel B 286 C.

Both compositions were tested by centrifuge at 3800 r.p.m. and anacceleration of 1590 g. Gel A showed no separation after 43 minutes,only 2% separation after 103 minutes, and 8% separation after 486minutes.

EXAMPLE V Other compositions having the desired cohesive shaperetentiveproperties were as follows:

NH NO (14,000 r.p.m. grind) NH ClO (1725 r.p.m. grind) butyl oxalatepolyvinyl chloride NH ClO (1725 r.p.m. grind) NH NO (14,000 r.p.m.grind) butyl oxalate polyvinyl chloride macro, 14,000 and 172s r.p.m.grinds, 2: 1 glyceryl (triacetylricinoleate) polyvinyl chloride It willbe noted, as exemplified by the aforedescribed compositions, that thesemi-solid monopropellants are formulated in such manner that they donot change significantly duringstorage for indefinitely long periods ofThe high density produced by inclusion of the solid oxidizer and, insome cases, additionally of a finely divided solid metal fuel, makespossible a high weight/volume loading ratio as compared withconventional mobile liquid propellant, and thereby reduced storage tankcapacity requirements or increased fuel capacity, in terms ofperformance, for. a storage chamber of given size.

The high autoignition temperature, low shockand impact-sensitivity,non-corrosiveness and non-toxicity, conferred by the inert, lowpour-point, high-boiling, liquid fuel, make the monopropellantcompositions safe to handle, to transportand .to store for extendedperiods of time under substantially any environmental temperatureconditions likely to be encountered. The stable gel or gel likecompositions do not leak. This is another important advantage ascompared with mobile liquids in terms of reduced fire and toxicityhazard and simplification of personnel and equipment precaution.

The unique physical characteristics of the monopropellant compositionsmake possible a new and highly advantageous method for generating gasesof high available energy'by extruding the material in the form of anydesired coherent shape into a combustion chamber and burning the leadingface of the continuously advancing shaped material. under stress atambient temperatures, the monopropellant can be fed into the combustionchamber at a rate adjusted to the desired mass burning rate of thecomposition so that at equilibrium or steady-state burning, namely whenthe mass burning rate does not vary with time, the burning surface ofthe continuously extruding propellant remains substantially stationaryrelative to the walls of the combustion chamber. Since burning isconfined to a welldefined burning-surface area, much as in the case ofthe burning of solid propellant grains, combustion chamber lengthrequirements are generally quite small, both as compared with thatneeded for complete reaction of sprayed or'atomized conventional mobileliquid propellants and for housing and combustion of conventional solidpropellant grains. This makes possible a substantial saving in deadweight, since the combustion chamber not only must be built to withstandthe high combustion gas pressures, but must also be heavily insulatedand made of materials, generally heavy, such as alloy steel or nickelalloys such as Inconel, which are resistant to the corrosive gases.Unlike solid propellant combustion chambers which must conform to designrequirements of the propellant grain, the combustion chamber for usewith my propellant compositions can be designed to meet the shape orother requirements of the particular gas generator device;

Burning surface area of the extruded shape-retaining monopropellant canbe predesigned and controlled by such means as varying the number, shapeand size of the injection orifices and by varying the rate of extrusionof the propellant into the combustion chamber. Thus, mass burning rateof the propellant and amount and pressure of combustion gases generatedcan easily be regulated by controlled feeding. In this way, the rate ofgas generation can be tailored to particular requirements both beforeand during operation within limits set by the particular properties ofthe monopropellant compositions and the structural limitations of therocket, gas generator or other device; Similarly, factors affectingburning rate of the propellant material, such as its ambient temperatureor pressure conditions in the combustion chamber can be compensated forby controlling feeding rate or adjust- Because of the fluidity of thematerial 10 ment of the size or shape of the mass of injected material.

Duration of combustion is limited only by the capacity of themonopropellant storage container and appropriate means for cooling thewalls of the combustionchamber and can be continuous or intermittent.Combustion can he quenched at any time by any suitable means such as acut-off device which closesthe injection orifice. Combustion can bereinitiated by opening the shut-oifmechanism and reigniting the leadingface of the extruding propellant.

The stable, uniform dispersion of the finely-divided solid oxidizer oroxidizer and solid metal fuel, ensures uniformity of burning rate at theconstantly generating burning surface as the end-burning materialadvances. This is of considerable importance since it assures a constantor controllable rate of gas generation.

The cohesiveness of the shape-retaining gel composition, furthermore,generally is sufliciently high to maintain integrity of the propellantunder conditions of vibration and acceleration against breaking-off orseparation of portions of the extruding mass into the combustionchamber. This is of importance not only for control of the desiredburning surface area but to avoid loss or wastage of unburned propellantin some applications, as, for example, rocket motors, by venting of thematerial out the nozzle under such conditions as high acceleration. Thisis frequently a problem in the case of the burning of atomized mobileliquid propellants, some unburned particles of which fly out the Venturinozzle.

Another advantage of the monopropellants stems from their substantialnon-fluidityexcept under stress since, unlike mobile'liquids, it makesthe system substantially immune to attitude. This makes unnecessaryelaborate precautions to maintain the stored propellant in constantcommunication with the feeding orifice into the combustion chamber.

Like conventional ambient liquid monopropellants, as distinguished fromliquid bipropellants, the system requires only one storage container orreservoir, one set of pressurizing means, feeding tubes and controlvalves, thereby simplifying the complexity of the device and reducingweight. There is also no need for combustion catalysts in the combustionchamber. Construction problems are further simplified bynon-corrosiveness of the fuel.

Thus, it will be seen that the heterogeneous monopropellants of myinvention combine the advantages of the conventional mobile liquidmonopropellants and solid propellants and eliminate most of theirdisadvantages. Like the solid propellants, the compositions arecharacterized by excellent stability, high density, low sensitivity toshock and impact, high auto-ignition temperatures, high specificimpulse, absence of leakage, excellent storageability andsystem-attitude tolerance. They are free from such defects of solidpropellants as the requirement of predetermined, set parameters, such aspredesigned shaping and size, venting and wasting of large amounts 'ofsurplus gases, limitation as to duration of the combustion cycle,inability to compensate for ambient temperature effect on burning rate,tendency to become brittle at low ambient temperatures which frequentlycauses fracturing, the dangers of mechanical flaws, impracticality ofreignition and intermittent action, and large combustion chamber size.

I claim:

1. In a heterogeneous monopropellent composition consisting essentiallyof a dispersion of finely-divided, insoluble, solid inorganic oxidizerin a continuous oxidizable organic fuel matrix which forms gaseouscombustion products, said organic fuel matrix consisting essentially ofa hydrocarbon compound consisting of carbon and hydrogen and an activeorganic compound which contains combined oxygen available for oxidationof other molecularly-combined components of said active compound, saidactive compound containing a radical selected from the group consistingof nitroso, nitro, nitrite and nitrate,

said oxidizer being present in amount. sufficientto maintain activecombustion of the hydrocarbon compound, said organic fuel matrixcontaining, in addition, from to a minor amount of a gelling agent, allof which is dissolved therein, the'improvement in which said organicfuel matrix, including said'dissolved gelling agent when present, is aliquid which is'rnobile at ordinary temperatur'es, comprises at leastabout 8% by weight of said composition and comprises at least two liquidcomponents all of which have a maximum vapor pressure of about 25 mm. Hgat 100 C., said liquid components consisting essentially of at least 50%by weight of said hydrocarbon compound, and from a minor amount up toabout 50% by weight of said active organic compound, each of saidhydrocarbon compound and said active organic compound being a liquid atordinary temperatures, said monopropellant being an extrudible,thixotropic composition which requires a finite stress to produce flow,is indefinitely capable, after storage, of continuous fiow at ambienttemperatures under a maximum shear stress at a wall of p.s.i., and has aminimum tensile strength of about 0.01 psi.

2. The-monopropellent composition of claim 1 in which the maximum shearstress at a wall is 1 p.s.i. I

3. The monopropellent composition of claim 1 in which the solid oxidizercomprises particles of different size.

4. The monopropellent composition of claim 3 inwhich the minimum tensilestrength of the composition is about 003 psi. and the solid oxidizercomprises particles distributed in size such that the minimum ratio ofthe largest andthe smallest particles is about 2: 1.

V 5. The monopropellent composition of claim 1 in which the oxidizer isa solid, inorganic, oxidizing salt.

6. The monopropellent composition of claim 1 in which said gellingagent, when present, is selected from the 3Z2 7 groupconsistingof'natural organic polymers, synthetic organic polymers, saltsof higher fatty acids, salts of naphthenic acid, and bentonite'clays.

7. The monopropellent composition of claim 6 in which the maximum shearstress at a wall is 1 psi.

8. Thernonopropellent composition of claim 6 in which the said oxidizercomprises particles of difierent size.

9. The monopropellent composition of claim 8, in which the solidoxidizer comprises particles of different size and the minimum tensilestrength of the composition is about 0.03 p.s.i.

10. The monopropellent composition of claim 6 in which the oxidizer isasolid, inorganic, oxidizing salt.

11. The monopropellent composition of claim 5 in which the oxidizer isammonium perchlorate.

12. The monopropellent composition of claim 10 in which the oxidizer isammonium perchlorate.

13. The monopropellent composition of claim 1 which contains in additiona timely divided metal fuel dispersed in said continuous matrix of saidliquid fuel.

14. The monopropellent compositionof claim 13 in which the maximum shearstress at'a wall is 1 p.s.i.

15. The monopropellent composition of claim 6 which contains in additiona finely divided metal fuel dispersed in said continuous matrix of saidliquid fuel.

16. The monopropellent composition of claim 13 in which the oxidizercomprises particles of different sizes.

17. The monopropellent composition of claim 13 in which the oxidizer isa solid, inorganic oxidizing salt.

18. The monopropellent composition of claim 13 in which the metal fuelis aluminum.

19. The monopropellent composition of claim 6 in which the geiling agentis polyvinyl chloride.

No references cited.

1. IN A HETEROGENEOUS MONOPROPELLENT COMPOSITION CONSISTING ESSENTIALLYOF A DISPERSION OF FINELY-DIVIDED, INSOLUBLE, SOLID INORGANIC OXIDIZERIN A CONTINUOUS OXIDIZABLE ORGANIC FUEL MATRIC WHICH FORMS GASEOUSCOMBUSTION PRODUCTS, SAID ORGANIC FUEL MATRIX CONSISTING ESSENTIALLY OFA HYDROCARBON COMPOUND CONSISTING OF CARBON AND HYDROGEN AND AN ACTIVEORGANIC COMPOUND WHICH CONTAINS COMBINED OXYGEN AVAILABLE FOR OXIDATIONOF OTHER MOLECULARLY-COMBINED COMPONENTS OF SAID ACTIVE COMPOUND, SAIDACTIVE COMPOUND CONTAINING A RADICAL SELECTED FROM THE GROUP CONSISTINGOF NITROSO, NITRO, NITRITE AND NITRATE, SAID OXIDIZER BEING PRESENT INAMOUNT SUFFICIENT TO MAINTAIN ACTIVE COMBUSTION OF THE HYDROCARBONCOMPOUND, SAID ORGANIC FUEL MATRIX CONTAINING, IN ADDITION, FROM 0 TO AMINOR AMOUNT OF A GELLING AGENT, ALL OF WHICH IS DISSOLVED THEREIN, THEIMPROVEMENT IN WHICH SAID ORGANIC FUEL MATRIX, INCLUDING SAID DISSOLVEDGELLING AGENT WHEN PRESENT, IS A LIQUID WHICH IS MOBILE AT ORDINARYTEMPERATURES, COMPRISES AT LEAST ABOUT 8% BY WEIGHT OF SAID COMPOSITIONAND COMPRISES AT LEAST TWO LIQUID COMPONENTS ALL OF WHICH HAVE A MAXIUMVAPOR PRESSURE OF ABOUT 25 MM. HG AT 100*C., SAID LIQUID COMPONENTSCONSISTING ESSENTIALLY OF AT LEAST 50% BY WEIGHT OF SAID HYDROCARBONCOMPOUND, AND FROM A MINOR AMOUNT UP TO ABOUT 50% BY WEIGHT OF SAIDACTIVE ORGANIC COMPOUND, EACH OF SAID HYDROCARBON COMPOUND AND SAIDACTIVE ORGANIC COMPOUND BEING A LIQUID AT ORDINARY TEMPERATURES, SAIDMONOPROPELLANT BEING AN EXTRUDIBLE, THIXOTROPIC COMPOSITION WHICHREQUIRES A FINITE STRESS TO PRODUCE FLOW, IS INDEFINITELY CAPABLE, AFTERSTORAGE, OF CONTINUOUS FLOW AT AMBIENT TEMPERATURE UNDER A MAXIMUM SHEARSTRESS AT A WALL OF 10 P.S.I., AND HAS A MINIMUM TENSILE STRENGTH OFABOUT 0.01 P.S.I.